IL-2 binds three transmembrane receptor subunits: IL-2Rβ and IL-2Rγ which together activate intracellular signaling events upon IL-2 binding, and CD25 (IL-2Rα) which serves to stabilize the interaction between IL-2 and IL-2Rβγ. The signals delivered by IL-2Rβγ include those of the P13-kinase, Ras-MAP-kinase, and STAT5 pathways.
T cells require expression of CD25 to respond to the low concentrations of IL-2 that typically exist in tissues. T cells that express CD25 include both FOXP3+ regulatory T cells (Treg cells), which are essential for suppressing autoimmune inflammation, and FOXP3− T cells that have been activated to express CD25. FOXP3− CD25+ T effector cells (Teff) may be either CD4+ or CD8+ cells, both of which may contribute to inflammation, autoimmunity, organ graft rejection, or graft-versus-host disease. IL-2-stimulated STAT5 signaling is crucial for normal T-reg cell growth and survival and for high FOXP3 expression.
In co-owned WO 2010/085495, we describe the use of IL-2 muteins to preferentially expand or stimulate Treg cells. When administered to a subject, the effect on Treg cells is useful for treating inflammatory and autoimmune diseases. Although the IL-2 muteins described therein are useful for expanding Treg over Teff cells in vivo, it was desirable to create IL-2 muteins that had optimal attributes for a human therapeutic.